1. Field of the Invention
The invention relates to an electromagnetic deflection unit for a cathode-ray tube comprising:
a hollow annular support having a longitudinal axis and a narrow and a wide end, PA0 a set of field deflection coils of the saddle type for field deflection of the CRT electron beam, these deflection coils being located on respective sides of a field coil separation plane through the longitudinal axis of the support and having a plurality of turns which mainly extend longitudinally of the support, and PA0 a set of line deflection coils of the saddle type for line deflection of the electron beam, PA0 the deflection coils of both sets being directly wound onto the support on the inner side thereof.
2. Description of the Related Art
Such a deflection unit is known from European Pat. No. EP 0 102 658 A1, which corresponds to U.S. Pat. No. 4,484,166, issued Nov. 20, 1984.
Cathode-ray tubes have a neck-shaped portion, in one end of which an electron gun is arranged and the other end of which passes into a conical portion which is joined by a screen. An electromagnetic deflection unit is arranged to surround this neck-shaped portion and to engage the conical portion. The narrow end of the support is located near the electron gun, while the wide end engages the conical portion of the cathode-ray tube.
Deflection units having coils directly wound onto the support afford great advantages, for example, that their manufacture requires a smaller number of assembling steps, that they can be wound with comparatively simple machines and that they are accurately reproducible. Such deflection units hitherto also have a disadvantage, however.
The deflection coils for line deflection produce a high-frequency magnetic field in the cathode-ray tube during operation. The sensitivity of these coils is strongly influenced by their distance from the cathode-ray tube. If they are located very close to this tube, a considerably smaller amount of energy need be dissipated in these coils for obtaining a certain magnetic field strength than if they are located close to the support and thus farther remote from this tube. This results in that, from a view point of energy consumption of the line deflection coils, it is favourable to first arrange the field deflection coils (which produce a low-frequency magnetic field and whose sensitivity is less dependent upon the distance from the cathode-ray tube) to engage the inner side of the support and then to provide the line deflection coils on the inner side of the field deflection coils.
In order to miniize Y coma errors, i.e. coma errors in directions at right angles to the direction of line deflection, it is of importance that the field deflection coils extend to an area closer to the electron gun than the line deflection coils. At the narrow end of the support, the field deflection coils consequently must extend in the direction of the longitudinal axis of the support beyond the line deflection coils. This results in that the line deflection coils must first be wound, directly against the inner side of the support, whereupon the field deflection coils mut be wound on the inner side thereof. In fact, if first the long field deflection coils should be wound onto the support, during the step of winding the shorter line deflection coils the winding wire would have to be continuously inserted between the turns of those longer coils. Such a method of winding practically cannot be carried out in mass production.
Consequently, two contrasting requirements are imposed on deflection units of the kind described.
For the sake of a high sensitivity of the deflection unit, more particularly of the line deflection coils, first the field deflection coils have to be wound against the inner side of the support and then the line deflection coils have to be wound;
for the sake of minimizing Y coma errors, the line deflection coils have to be wound against the inner side of the support and then the field deflection coils have to be wound.